Load-carrying vehicles, such as underground mining trucks, include suspension systems which absorb bumps and other terrain inconsistencies. Such vehicles are often heavily loaded with materials, such as ore or rock, greatly increasing their effective weight. For example, some load-carrying vehicles carry their own weight or more in materials. This results in the vehicle having drastically different effective vehicle weights in a loaded state than an unloaded state. Load-carrying vehicles are frequently driven in each of these states, which makes tuning the vehicle's suspension system difficult.
Some suspension systems of load-carrying vehicles are tuned according to the loaded state to prevent bottoming out, which could damage the vehicle. Bottoming out occurs when the suspension reaches its fully compressed state, resulting in damaging impact to various components of the system, such as the shock absorbers or struts. This approach, however, results in an overly stiff response when the vehicle is driven in the unloaded state. As a result, terrain variations and inconsistencies create a rough and jarring ride for the vehicle operator in the unloaded state, reducing the operator's comfort and productivity.
To address these issues, some vehicle suspension systems adjust stiffness and ride height between the loaded and unloaded states. One such system is described in U.S. Pat. No. 6,959,932 which issued to Svartz et al. on Nov. 1, 2005 (“the '932 patent”). The suspension system of the '932 patent measures both the pressure in an air spring and the load on a rear axle. In response to these measurements, the system adjusts the pressure in the air spring to change both the vehicle's ride height and the spring's stiffness. To adjust for the loaded state, air is pumped into the system at a pressure greater than the system's operating pressure, increasing both the ride height and stiffness. Then to adjust for the unloaded state, a valve releases air from the air spring to reduce its stiffness and the vehicle's ride height.
Another vehicle suspension system is described in U.S. Pat. No. 8,075,002 which issued to Pionke et al. on Dec. 13, 2001 (“the '002 patent”). The suspension system uses a dedicated hydraulic pump to force fluid from a reservoir into the suspension system and increase its operating pressure, thereby increasing its stiffness. The hydraulic pump must be capable of overcoming the system's working pressure, which can be substantial, to force hydraulic fluid into the system. For example, the hydraulic pump should preferably be capable of producing 9,000 pounds per square inch (psi) or more.
Although the systems of the '932 patent and '002 patent may help to improve the vehicle suspension system's characteristics, they are energy inefficient and result in undesirable delays when switching between operating modes. The air supply of the '932 patent and the hydraulic pump of '002 patent consume power each time they increase the system's stiffness to switch from the unloaded mode to the loaded mode. Additionally, switching from the unloaded to loaded mode requires waiting for the hydraulic pump to increase the system pressure to the desired pressure, adding undesirable delay. Lastly, the system of the '002 pump also requires an oil reservoir, an additional component adding complexity and cost to the system.
The disclosed suspension system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.